The present invention relates to a method and apparatus for measuring the developing ability of a powdered developer for electrostatography. Typical applications are in electrostatic copying machines, electrostatic printers for computers and facsimile systems and the like.
In electrostatography an electrostatic image is formed on a dielectric material through photoconduction, electrostatic discharge or the like. A powdered or liquid developer is applied to the material and adheres to the areas of high electrostatic charge to form a toner image. The toner image is transferred and fixed to a copy sheet to provide a permanent reproduction. Alternatively, the toner image may be fixed to the material on which the electrostatic image is orginally formed.
A powdered developer generally comprises two components which are mixed together. The image is actually developed by toner particles of small size which are colored black or another suitable color. Magnetic or non-magnetic carrier particles are mixed with the toner particles to aid in application of the toner particles to the material carrying the electrostatic image.
The carrier particles are generally larger in size than the toner particles and are higher or lower in the triboelectric series than the toner particles. Agitation of the developer causes the carrier particles and toner particles to rub together and produce opposite electrostatic charges which cause the toner particles to adhere to the carrier particles and also to the electrostatic image on the material for development. The carrier particles are not consumed in the developing process, as are the toner particles, but are recovered and recycled.
The carrier particles which are recovered are mixed with the developer and used again in the developing process. However, since toner was consumed, the ratio of toner to carrier particles progressively decreases. For this reason, it is necessary to periodically add additional toner to maintain the toner to carrier ratio at the desired value. If this ratio drops significantly, the density of the developed image will drop by a corresponding amount. The ratio of toner to carrier particles in the developer is known in the art as the toner density.
However, factors other than toner density act to vary the developed image density. The amount of induced electrostatic charge in the developer, the shapes of the toner and carrier particles, the degree of adherence of the toner particles to the carrier particles, the amount of deteriorated carrier particles, the ambient temperature and humidity all act as variables to affect the image density.
The developing ability of a developer for electrostatography is thus the sum total of all variables including those enumerated above and may be defined as the amount of toner which adheres to an electrostatically charged surface of unit area and unit electrostatic charge. Due to deterioration of carrier particles, for example, the developing ability and thereby the image density may decrease even though the toner density remains constant or even increases.
Numerous methods have been proposed in the prior art for measuring the toner density and developing ability of a developer for electrostatography. Japanese patent publication No. 38-17245 teaches how to color toner and carrier particles with different colors and determine the toner density by sensing the relative color of the developer. A drawback is that the carrier particles must be colored, which is not practical in various systems.
Another prior art method is disclosed in Japanese patent publication No. 46-8280 in which a developer comprises ferromagnetic carrier particles and non-magnetic toner particles. The permeability of the developer is sensed by a coil and corresponds to the toner density. This method, however, cannot be applied to developers in which the carrier particles are nonmagnetic, such as developers for cascade development systems. In addition, these two methods measure the toner density, rather than the developing ability, and do not reflect the actual change in developing ability which takes place due to deterioration of carrier particles or variations in ambient temperature and humidity.
Another known method is disclosed in U.S. Pat. No. 3,399,652 in which developer is applied to a rotating metal disc and the amount of adhered toner is measured by means of a light source and photosensor which sense light reflected from the disc. An electric potential is applied to the disc to urge toner to adhere thereto. This method measures developing ability rather than toner density. However, it is disadvantageous since it requires a disc and motor for rotating the same. In addition, the results are unreliable since the photosensor and light source become contaminated with toner.
Another method is disclosed in U.S. Pat. No. 3,727,065 in which developer is caused to flow between two parallel transparent electrodes. An electric potential is applied across the electrodes which causes toner to adhere to one of the electrodes. A light source and photosensor measure the amount of light transmitted through the electrodes and thereby the amount of adhered toner which corresponds to the developing ability. The main problem with this system is that it is impossible to base the measurement on a reference level which occurs when the electrodes are completely free of toner. The reference level varies as the result in changes of the amount of contamination of the electrodes with residual toner or dirt, variations in the output of the light source and voltage source for the electrodes, temperature variations of electrical components and the like. Although the potential is periodically reversed so that one electrode is cleaned while toner adheres to the other electrode, the reference level cannot be measured since toner always adheres to one or the other of the electrodes.
Another problem in the last described method is that the accumulation of toner on the electrodes tends to become excessive and a powerful light source and sensitive photosensor are reqiured to measure the toner accumulation. The result is high production cost and power requirements in addition to a generally inaccurate measurement. In addition, the uniform electric field across the electrodes does not accurately simulate electrostatic reproduction of printed documents since the relatively this lines making up the characters create uneven electric fields.
An improved apparatus is disclosed in Japanese patent publication No. 50-894l which comprises an additional photosensor and light source for measuring transmitted light through electrodes which are not energized with electric potential and used for the developing ability measurement. Although this method provides a reference level, the reference level is not accurate due to differences in the electrical characteristics of the photosensors and light sources and since it cannot measure the amount of residual toner which remains on the measurement electrodes after the cleaning step.
Another method is disclosed in Japanese patent publication No. 52-13935 which uses a chopper wheel to alternatingly uncover a reference density surface and a surface on which toner has adhered and direct light transmitted therefrom to photosensors which produce A.C. output signals. A smiilar apparatus is disclosed in Japanese patent publication No. 52-33734 which uses a single photosensor and means for storing a signal corresponding to reflected light from the reference density surface or toner adhered surface for comparison. Both of these methods involve relatively expensive mechanical components and provide results of insufficient accuracy.
Yet another prior art method is disclosed in Japanese patent publication No. 48-95243 in which developer is caused to fall onto and flow down an inclined transparent plate. A light source and photosensor measure the amount of toner which accumulates on the plate. This method is inaccurate in that it does not measure the actual developing ability and the toner tends to melt and stick to the plate, thereby further degenerating the accuracy of the reading.
Two color copying machines have been introduced for business applications. Typically, such a copying machine will produce copies in black and red, the most common colors used in business documents. Debit entries are made in accounting ledgers in red whereas credit entries are made in black. In addition, important parts of documents are often underlined in red.
In a two color copying machine a bipolar electrostatic image is formed corresponding in polarity to the two colors, and black and red toners of opposite electrostatic polarity are applied to the image for development. The toners may be applied separately or as a mixture. In the latter case, it is necessary to measure the developing ability of the toners separately since the black toner is consumed faster than the red toner. This is because most printing is in black. No methods proposed heretofore are capable of independently measuring the developing abilities of two toners mixed together with a carrier to constitute a two color, three component developer.